The Notch signaling pathway is one of several critical regulators of embryonic pattern formation, post-embryonic tissue maintenance, and stem cell biology. More specifically, Notch signaling is involved in the process of lateral inhibition between adjacent cell fates and plays an important role in cell fate determination during asymmetric cell divisions. Unregulated Notch signaling is associated with numerous human cancers where it can alter the developmental fate of tumor cells to maintain them in an undifferentiated and proliferative state (Brennan and Brown, 2003, Breast Cancer Res. 5:69). Thus carcinogenesis can proceed by usurping homeostatic mechanisms controlling normal development and tissue repair by stem cell populations (Beachy et al., 2004, Nature 432:324).
The Notch receptor was first identified in Drosophila mutants with haploinsufficiency resulting in notches at the wing margin, whereas loss-of-function produces an embryonic lethal “neurogenic” phenotype where cells of the epidermis switch fate to neural tissue (Moohr, 1919, Genet. 4:252; Poulson, 1937, PNAS 23:133; Poulson, 1940, J. Exp. Zool. 83:271). The Notch receptor is a single-pass transmembrane receptor containing numerous tandem epidermal growth factor (EGF)-like repeats and three cysteine-rich Notch/LIN-12 repeats within a large extracellular domain (Wharton et al., 1985, Cell 43:567; Kidd et al., 1986, Mol. Cell. Biol. 6:3094; reviewed in Artavanis et al., 1999, Science 284:770). Four mammalian Notch proteins have been identified (Notch1, Notch2, Notch3, and Notch4), and mutations in these receptors invariably result in developmental abnormalities and human pathologies including several cancers as described in detail below (Gridley, 1997, Mol. Cell. Neurosci. 9:103; Joutel & Tournier-Lasserve, 1998, Semin. Cell Dev. Biol. 9:619-25).
Notch receptors are activated by single-pass transmembrane ligands of the Delta, Serrated, Lag-2 (DSL) family There are five known Notch ligands in mammals: Delta-like 1 (DLL1), Delta-like 3 (DLL3), Delta-like 4 (DLL4), Jagged 1 (JAG1) and Jagged 2 (JAG2) characterized by a DSL domain and tandem EGF-like repeats within the extracellular domain. The extracellular domain of the Notch receptor interacts with that of its ligands, typically on adjacent cells, resulting in two proteolytic cleavages of Notch, one extracellular cleavage mediated by an ADAM (A Disintegrin And Metallopeptidase) protease and one cleavage within the transmembrane domain mediated by gamma secretase. This latter cleavage generates the Notch intracellular domain (ICD), which then enters the nucleus where it activates the CBF1, Suppressor of Hairless [Su(H)], Lag-2 (CSL) family of transcription factors as the major downstream effectors to increase transcription of nuclear basic helix-loop-helix transcription factors of the Hairy and Enhancer of Split [E(sp1)] family (Artavanis et al., 1999, Science 284:770; Brennan and Brown, 2003, Breast Cancer Res. 5:69; Iso et al., 2003, Arterioscler. Thromb. Vasc. Biol. 23:543). Alternative intracellular pathways involving the cytoplasmic protein Deltex identified in Drosophila may also exist in mammals (Martinez et al., 2002, Curr. Opin. Genet. Dev. 12:524-33), and this Deltex-dependent pathway may act to suppress expression of Wnt target genes (Brennan et al., 1999, Curr. Biol. 9:707-710; Lawrence et al., 2001, Curr. Biol. 11:375-85).
Mammalian Notch receptors undergo cleavage to form the mature receptor and also following ligand binding to activate downstream signaling. A furin-like protease cleaves the Notch receptors during maturation to generate juxtamembrane heterodimers that comprise a non-covalently associated extracelluar subunit and a transmembrane subunit held together in an auto-inhibitory state. Ligand binding relieves this inhibition and induces cleavage of the Notch receptor by an ADAM-type metalloprotease and a gamma-secretase, the latter of which releases the intracellular domain (ICD) into the cytoplasm, allowing it to translocate into the nucleus to activate gene transcription. Cleavage by ADAM occurs within the non-ligand binding cleavage domain within the membrane proximal negative regulatory region.
Hematopoietic stem cells (HSCs) are the best understood stem cells in the body, and Notch signaling is implicated in their normal maintenance as well as in leukemic transformation (Kopper & Hajdu, 2004, Pathol. Oncol. Res. 10:69-73). HSCs are a rare population of cells that reside in a stromal niche within the adult bone marrow. These cells are characterized both by a unique gene expression profile as well as an ability to continuously give rise to more differentiated progenitor cells to reconstitute the entire hematopoietic system. Constitutive activation of Notch1 signaling in HSCs and progenitor cells establishes immortalized cell lines that generate both lymphoid and myeloid cells in vitro and in long-term reconstitution assays (Varnum-Finney et al., 2000, Nat. Med. 6:1278-81), and the presence of Jagged1 increases engraftment of human bone marrow cell populations enriched for HSCs (Karanu et al., 2000, J. Exp. Med. 192:1365-72). More recently, Notch signaling has been demonstrated in HSCs in vivo and shown to be involved in inhibiting HSC differentiation. Furthermore, Notch signaling appears to be required for Wnt-mediated HSC self-renewal (Duncan et al., 2005, Nat. Immunol. 6:314).
The Notch signaling pathway also plays a central role in the maintenance of neural stem cells and is implicated in their normal maintenance as well as in brain cancers (Kopper & Hajdu, 2004, Pathol. Oncol. Res. 10:69-73; Purow et al., 2005, Cancer Res. 65:2353-63; Hallahan et al., 2004, Cancer Res. 64:7794-800). Neural stem cells give rise to all neuronal and glial cells in the mammalian nervous system during development, and more recently have been identified in the adult brain (Gage, 2000, Science 287:1433-8). Mice deficient for Notch1; the Notch target genes Hes1, 3, and 5; and a regulator of Notch signaling presenilin1 (PS1) show decreased numbers of embryonic neural stem cells. Furthermore, adult neural stem cells are reduced in the brains of PS 1 heterozygote mice (Nakamura et al., 2000, J. Neurosci. 20:283-93; Hitoshi et al., 2002, Genes Dev. 16:846-58). The reduction in neural stem cells appears to result from their premature differentiation into neurons (Hatakeyama et al., 2004, Dev. 131:5539-50) suggesting that Notch signaling regulates neural stem cell differentiation and self-renewal.
Aberrant Notch signaling is implicated in a number of human cancers. The Notch1 gene in humans was first identified in a subset of T-cell acute lymphoblastic leukemias as a translocated locus resulting in activation of the Notch pathway (Ellisen et al., 1991, Cell 66:649-61). Constitutive activation of Notch1 signaling in T-cells in mouse models similarly generates T-cell lymphomas suggesting a causative role (Robey et al., 1996, Cell 87:483-92; Pear et al., 1996, J. Exp. Med. 183:2283-91; Yan et al., 2001, Blood 98:3793-9; Bellavia et al., 2000, EMBO J. 19:3337-48). Notch1 point mutations, insertions, and deletions producing aberrant Notch1 signaling have also been found to be frequently present in both childhood and adult T-cell acute lymphoblastic leukemia/lymphoma (Pear & Aster, 2004, Curr. Opin. Hematol. 11:416-33).
The frequent insertion of the mouse mammary tumor virus into both the Notch1 and Notch4 locus in mammary tumors and the resulting activated Notch protein fragments first implicated Notch signaling in breast cancer (Gallahan & Callahan, 1987, J. Virol. 61:66-74; Brennan & Brown, 2003, Breast Cancer Res. 5:69; Politi et al., 2004, Semin. Cancer Biol. 14:341-7). Further studies in transgenic mice have confirmed a role Notch in ductal branching during normal mammary gland development, and a constitutively active form of Notch4 in mammary epithelial cells inhibits epithelial differentiation and results in tumorigenesis (Jhappan et al., 1992, Genes & Dev. 6:345-5; Gallahan et al., 1996, Cancer Res. 56:1775-85; Smith et al., 1995, Cell Growth Differ. 6:563-77; Soriano et al., 2000, Int. J. Cancer 86:652-9; Uyttendaele et al., 1998, Dev. Biol. 196:204-17; Politi et al., 2004, Semin. Cancer Biol. 14:341-7). Evidence for a role Notch in human breast cancer is provided by data showing the expression of Notch receptors in breast carcinomas and their correlation with clinical outcome (Weijzen et al., 2002, Nat. Med. 8:979-86; Parr et al., 2004, Int. J. Mol. Med. 14:779-86). Furthermore, overexpression of the Notch pathway has been observed in cervical cancers (Zagouras et al., 1995, PNAS 92:6414-8), renal cell carcinomas (Rae et al., 2000, Int. J. Cancer 88:726-32), head and neck squamous cell carcinomas (Leethanakul et al., 2000, Oncogene 19:3220-4), endometrial cancers (Suzuki et al., 2000, Int. J. Oncol. 17:1131-9), and neuroblastomas (van Limpt et al., 2000, Med. Pediatr. Oncol. 35:554-8), suggestive of a potential role for Notch in the development of a number of neoplasms. Notch signaling may play a role in the maintenance of the undifferentiated state of Apc-mutant neoplastic cells of the colon (van Es & Clevers, 2005, Trends in Mol. Med. 11:496-502).
The Notch pathway is also involved in multiple aspects of vascular development including proliferation, migration, smooth muscle differentiation, angiogenesis and arterial-venous differentiation (Iso et al., 2003, Arterioscler. Thromb. Vasc. Biol. 23:543). Furthermore, DLL1-deficient and Notch2-hypomorphic mice embryos show hemorrhaging that likely results from poor development of vascular structures (Gale et al., 2004, PNAS, 101:15949-54; Krebs et al., 2000, Genes Dev. 14:1343-52; Xue et al., 1999, Hum. Mel. Genet. 8:723-30; Hrabe de Angelis et al., 1997, Nature 386:717-21; McCright et al., 2001, Dev. 128:491-502).
Accordingly, there is a need in the art for therapeutic antibodies that bind to Jagged-1 (JAG1) epitopes and could be used as therapeutic agents for treating Notch-signaling tumors, including breast, prostate, colorectal, lung and other solid tumors.